Modern potable water supply systems and waste water handling systems have long relied on various methods to remove microbiological contamination. In treatment of water, it is generally contamination by fecal coliforms, particularly E. coli bacteria, that is monitored and treated as the presence of these organisms indicates fecal contamination that can lead to rapid spread of disease through the water supply or receiving water body. It is still not uncommon to discharge waste water streams without any treatment at all, raising the question of increased ecological damage and fouling of water bodies and potable water supplies. Generally, in the past, the preferred method of eliminating or reducing microorganism contamination in potable water supplies and in waste water handling systems where practised has been through the use of chlorination. Chlorination suffers from disadvantages in that it is expensive to administer, the dosage of chlorine must be carefully controlled to ensure proper and effective treatment of the water, and it is not particularly effective with water that contains a large amount of suspended particulate solids. In treating waste water streams, there is the potential of introducing harmful and carcinogenic organo-chlorides.
Other methods have been developed to treat microbiological contamination of water that do not involve the addition of a treatment agent. The most popular amongst these techniques is exposing water to ultra-violet radiation to kill any foreign organisms. A number of systems have been developed to deliver ultra-violet radiation to a liquid.
When designing a system to deliver ultra-violet radiation to water, there are a number of factors to take into account. First, ultra-violet radiation, particularly when of low intensity, does not penetrate very deeply into water and therefore, it is necessary to expose a relatively thin film of water to the radiation to ensure that a lethal dose of radiation is delivered. Second, the effectiveness of ultra-violet radiation is reduced when treating murky water such as sewage effluent. Suspended, particulate solids that increase the turbidity of the water cannot be penetrated by the ultra-violet radiation.
The result of the foregoing design considerations is that previous ultra-violet water treatment systems have often been small scale systems that have a limited throughput. Examples of prior small scale systems for treating aquarium water or water as it emerges directly from the household tap are exemplified in U.S. Pat. No. 2,338,387 to Whitman, U.S. Pat. No. 3,731,090 to Veloz, and U.S. Pat. No. 4,184,076 to Kosnoff.
U.S. Pat. No. 4,336,223 to Hillman, U.S. Pat. No. 3,711,709 to Rudolf and U.S. Pat. No. 3,837,800 to Wood disclose larger scale systems for treating water with ultra-violet radiation. Wood is a good example of the design considerations that limit current ultra-violet sterilization systems. Wood discloses a system that is limited to thin sheet flow of water to be treated past an ultra-violet source to ensure adequate penetration of the ultra-violet radiation. To increase the dosage of radiation to a level that will kill microorganisms, Wood has to use a long residence time. The use of laminar sheet flow of water with a long residence time leads to a very limited capacity for Wood's system.